1. Technical Field of the Invention
The present invention relates to an apparatus for thermally protecting a controller for on-vehicle alternators, and in particular, to the apparatus for protecting the controller from heat generated from a switching circuit incorporated in the controller.
2. Related Art
A power switching device is incorporated in a variety of types of machines, and one type of such a device is an excitation-current control apparatus (called “regulator”) for an alternator mounted in a vehicle. Such a power switching device needs fast switching operations, thereby generating a large amount of heat. Thus, the power switching device is subjected to a rise in its temperature when an ambient temperature is higher or when a cooling apparatus cooling down the power switching device does not work well.
To measure the above heat generation, it is usual that thermal protection control is executed to suppress heat generated from power switching components. This thermal protection control is carried out by detecting the temperature of the power switching components, and limiting an electric current passing through the components (i.e., activating a current-limiting operation) when the detected temperature exceeds an activating threshold temperature Tactivate. In contrast, this current-limiting operation is deactivated when the detected temperature becomes lower than a deactivating threshold temperature Tdeactivate. This deactivating threshold temperature is set to be lower than the activating threshold temperature.
It is normal that the current-limiting operation is performed by turning off the power switching components. Alternatively the current-limiting operation may be performed by effective current control (e.g., on-duty ratio control of the power switching components) to keep the target effective current, passing through the components, lower than a predetermined average current.
This thermal protection control is proposed by, for instance, U.S. Pat. No. 6,995,544, which is widely employed by conventional power switching devices composed of power switching components which are required to operate within a strictly restricted temperature range.
However, as mentioned above, in the conventional thermal protection control, the deactivating threshold temperature at which the current-limiting operation is deactivated is set lower than the activating threshold temperature at which the current-limiting operation is activated. That is, the conventional thermal protection control can be called “normal hysteresis control (refer to FIG. 4A).” Further, the activating threshold temperature is usually set to an uppermost temperature, so that in a temperature range up to the uppermost temperature the components are assured to operate without malfunction.
Under this combination of the activating and deactivating threshold temperatures, in the temperature decreasing cycle, the power switching components keep their turned-off states until the detected temperature of the components becomes lower than the deactivating threshold temperature, although the components are able to already operate without malfunction in a temperature range from the activating threshold temperature down to the deactivating threshold temperature.
In other words, the power switching components are not turned on until the detected temperature of the components or the device reaches the deactivating threshold temperature which is set at the uppermost temperature, thus being laid idle in the temperature range between the activating and deactivating threshold temperatures. In the temperature range between the activating threshold temperature Tactivate and the deactivating threshold temperature Tdeactivate, the components can operate normally but lays idle. Resultantly, in the temperature decreasing cycle, the cumulative output of the alternator is reduced.
On the contrary, considering when the power switching component turns off (i.e., when the current-limiting operation is activated; in the temperature increasing cycle), the power switching component is not turned off until the detected temperature of the component reaches the uppermost temperature (i.e., the activating threshold temperature Tactivate in the normal hysteresis control), thereby experiencing a relatively severe temperature environment.
Additionally, there is a drawback that the output current of the alternator is abruptly changed when the power switching component is turned on/off in executing the current-limiting control. Concurrently, when this abrupt change in the output current occurs, a mechanical impulse (i.e., a torque shock) affecting the engine occurs. This impulse is caused by abrupt torque changes of the alternator when the component controlling an excitation current of the alternator turns on/off in the regulator.
Further, to suppress the thermal generation at the power switching component, the conventional thermal protection control, which is performed by the regulator, operates to lower the threshold voltage of the component by 1 or 2 volt depending on the output voltage of the battery, when the temperature of the regulator exceeds the activating threshold temperature so as to reduce the ON-duty current passing the component.
After this, the changed threshold voltage is returned to an initial threshold voltage before executing the above operation, when the temperature of the regulator is lower than the deactivating threshold temperature. This recovery control increase an additional ON-duty current passing the component, which increases an additional torque of the alternator, thus accelerating the torque shock.